Of Chimney Fireplaces. 495 



placed. In determining this point, there are many 

 things to be considered, and several advantages and dis- 

 advantages to be weighed and balanced. 



As the smoke and vapour which ascend from burning 

 fuel rise in consequence of their being rarefied by heat, 

 and made lighter than the air of the surrounding at- 

 mosphere ; and as the degree of their rarefaction, and 

 consequently their tendency to rise, is in proportion to 

 the intensity of their heat ; and further, as they are 

 hotter near the fire than at a greater distance from it, it is 

 clear that the nearer the throat of a chimney is to the 

 fire, the stronger will be what is commonly called its 

 draught^ and the less danger there will be of its smoking. 

 But on the other hand, when the draught of a chimney 

 is very strong, and particularly when this strong draught 

 is occasioned by the throat of the chimney being very 

 near the fire, it may so happen that the draught of air 

 into the fire may become so strong as to cause the fuel 

 to be consumed too rapidly. There are likewise several 

 other inconveniences which would attend the placing of 

 the throat of a chimney very near the burning fuel. 



In introducing the improvements proposed, in chim- 

 neys already built, there can be no question in regard to 

 the height of the throat of the chimney,^for its place 

 will be determined by the height of the mantle. It can 

 hardly be made lower than the mantle; and it ought 

 always to be brought down as nearly upon the level with 

 the bottom of it as possible. If the chimney is apt to 

 smoke, it will sometimes be necessary either to lower 

 the mantle or to diminish the height of the opening of 

 the fireplace, by throwing over a flat arch, or putting in 

 a straight piece of stone from one side of it to the 

 other, or, which will be still more, simple and easy in 



